Single side band transmission



March 12, 1935. E, s PuRlNGToN 1,994,048

SINGLE SIDE BAND TRANSMISSION Filed Sept. 6, 1950 2 Sheets-Sheet 1 ELLlSON PUNNGTON wv-C N N ATTORNEY March 12, 1935. E. s. PURINGTON 1,994,048

SINGLE SIDE BAND TRANSMISSION Filed Sept. 6, 1930 2 Sheets-Sheet 2 I MI mxxmxx w INVENTOR o ELLlSON S. FURINGTON a a) BY 7% ATTORNEY hi'fj Other" bjects Patented Mar. 12, 1935 swam-sinemmrnresmssm 8 Glaims.

"The present invention relates tothe'producltion and radiation of single side band transmission of radio telephonic signals or transmission of telephonio'sign'als by a single side'band in com-' 5 loinatioriwith' a carrier.

I It 'also'relates to an-improved'method of'producing' two phase audio frequency'currentsfor in such radio telephonic communication, and alsofor otherpurposes. Anobject of this invention is toreduce interx ference by lessening the band necessary for transmission of a given number of radio signals. '-'-Another object of this invention is to produce two currents of the same audio frequency and --signal strength'at a predeterminedphase displacement regardless of the "frequency.

- Still another'ob'ject of this invention is to re ducethelbandi Width required for transmission of intelligence, thereby increasing the number of channels-possibleior communication.

fects, by permitting the -carrier 'wave at there *ceiver'to be demodulated vwith'one' side band only.

A-still-further object isthe-minimizing of;fad-" '25- ing by providingconstant' signalstrengthof the -;-;;carrier atthe receiver. v W

additional object-of this invention is; the improvement of transmission efiiciencyywhereby wtheradiated power: required for communication 30 maybe lessened. v V H g =Anotherobject is the improvement of efliciency f of" signaling by eliminating carrier frequency fromthe: tr l th eb p t in h e stregth of side bands representative of the signal being conveyed.

Still a furtherobject of this invention isto.

produce two currents of the same audio frequency;

" n e n 'l i h a p ed termined Ph displacement for each constituent frequency rewlcga dless o'fth' frequency. L H; n further object is the improvement of quality "f trahSIfiiSS iOhby lirilil l 'ltil'l gth llildsil d @111- M ts produced by "beati 'thebth'er sidband. I v v I v and'uses of thisi'nveiitiorrvvill appear from the followingdescription takenin ntiectionwith'thejaccompanyingidrawings, I

quired for transmission,- 'and lessened interference between; channels, Ehe tr sion 'by 'SingIeside band only requiresthe"other higlifirequencyphase by the "otherlow ng of onesid band'vvitlr 7 The transmission of signals" b'y'""use of carrier and single 'sideband is an effective substitute for? '50 theus'ual carrier with both side bands;' wh ich resultsinlessenedfading effects; leSs banQ'vv idtLrrevolVesT'first; producing eftwo pliasd'rriod ating" 5 current; second, producing a two phase rent tobem'odulated; tmmmoumatgngeue 1gh1re- T quenc'y phase with one 10wrrequency pnasiand frequency" phase? by 1' push pull metli ods fourth; "combiningfthe re's'ultinfg' modulate urrents 'additively' OrsubtractiveIy' togiv'e side band feither higher or lowe'rthan thecelrrier "wave. r

For transmission of a carrierand asingle 'sid' 15 "band; the process" isthe' *same" except *that' the carrier is "also 'transmittedf'by-"'adding"carrier *energy at some part of 'itheproc'ess.

Intheprocess of pi'oducingtwo phase audio current an original :single" phase "audi'o" current? 20 r as produced in the usual microphone 'pick up and amplifier circuit, -is convertedtoa high frequency single side :ba'nd': by push-pull modulatiorrnnd {filtering out the? undesired frequencies. A'I'tWQ phase current ofjthe "carrierfrequency used ifi" the;'-push-pul1-modulation is produced; and" half the energy of "the single side'iband is'co'mbined withcurrent of one phase; and'thepthei' half is "combined with-current of the Tether phase. The I resulting wave forms are separately demodulated} producing two'audio bands. of the 'sa'me'stren'gths for each'freguency component-but withquadrature phase difference, p r 7 Carrier and' one sidebandi'are'all 'that are-required H for good transmission Inclusion of the other 'side band increases the signalcconveyed. but requires additional; band; width? When" the me'diumbetween; transmitter and" receiveroperates abnormally, often there is interferencebetween theeff ect'of carrier-end one :sideb'an'd and 4 v theeitect of carrier and the 'other side band,ii'esulting in bad distortion. Also, the efi'ect'ofone side band; and the other side band produoesf distgirtion under-g perfect transmission conditions.

There is some loss of transmission;1efiiciencf45 s ikll id 'X fli$9eid n 9; .e..b. nd 1Wh Q T -Y be m d 1 i ar by in r asi the nerg f the larger amount of energy which can be emitted in a side band if the carrier is omitted greatly increases the transmission efiiciency. Further, single side band transmission is even more free from fading than carrier and single side band transmission, because the carrier at least may be made steady at the receiver. As an alternative, the carrier may be made to increase or decrease in accordance with decreases or increases of received side band energy to maintain signal strength further constant. Further, with single side band only, no sustained heterodyne tones are produced, which is perhaps the worst source of interference produced between two transmitting stations on the same channel.

It is to be understood that the carrier and single side band transmission may be used with the usual type receivers, but the single side band system requires special receiving equipment.

With regard to interference, it is quite evident that the side frequencies of one transmission must be spaced from the side frequencies of another transmission to permit selection in reception. It is also evident that when both side frequencies are transmitted the spacing of carrier frequencies for a plurality of transmissions is necessarily about twice the maximum audio frequency to be expected. ,With one side band eliminated, a single carrier could be utilized to transmit two audio frequency signals or a single side band could be transmitted with each of two carriers spaced half 7 the distance normally required. In this case the tuning would be to some point between the carrier and the extreme limit of the side frequency band.

Heretofore the principal difliculty in eliminating one of the side bands transmitted has been that a filter which is broad enough to cover without substantial diminution of transmission over It is the object of this invention to provide for a system for eliminating completely one of the side frequencies which would otherwise be radiated, by neutralizing the undesired side frequency components by production of this side frequency component over two systems in phase opposition, however, simultaneously producing the desired side band components over the two systems in phase. This'prevents then the necessity of filtering out the undesired side frequencies as they are never produced. All filters used operate at frequencies such that filter design for proper cut-off is easily made, with no loss of tonal qualities.

Having thus briefly described my invention, attention is invited to the accompanying figures in which;

Fig. l is a diagram of a circuit which is adapted to produce two phase currents of audio frequencies;

Fig. 2 is a circuit for modulating a two phase carrier frequency by the two phase audio frequency currents produced by the circuit of Fig. l, and thus producing and transmitting a single side. band or a single side band and the carrier frequency.

which shows a circuit for producing two audio frequency currents at phase quadrature, the audio frequency current from which the two phase current is to be produced is supplied through terminals l and 2 and a high frequency auxiliary current is supplied by source 3. The audio frequency current is impressed upon the grid of push-pull modulator devices 4 and 5.

The auxiliary current is amplified by the device 6 and impressed into a closed circuit including inductance 7, condenser 8, inductance 9 and resistor 10 shunted by the condenser 11.

- The inductance 7 is coupled to the plate circuit of the push-pull modulator 45, and is thus adapted to impress the high frequency auxiliary current upon the resistors 12 and 13 included in thisplate circuit. The operation of the devices 4 and 5 as push-pull modulators is described in my copending application, Serial No. 365,222, filed May 18, 1929. In this application is also described the details of the use of condensers l5, l6, l7 and 18 in obtaining perfect elimination of the high frequency output of the push-pull modulator.

The resistor 19 and filter 20 are so proportioned as to eliminate the lower side band produced by the push-pull modulation leaving energy of the upper side band which later is impressed upon the grids of devices 22 and 23 in parallel.

Inductance 9 and resistor 10, shunted by the high impedance condenser 11, are so chosen as to make the voltage drop across 9 equal to, but in phase quadrature with, the voltage drop across the resistor 10. Thus a two phase voltage is impressed upon the grids of the tubes 24 and 25 respectively.

The four tubes, 22, 24, 23, and 25 are so polarized that they exhibit plate rectification wherefore audio tone currents will exist across the resistors 26 and 27 included in the plate circuit of each of these devices respectively. But, because of the differences of phase of the grid voltages of 24 and 25, the beat current produced in resistor 26 will be in phase quadrature to that produced in resistor 27. The output currents of the detection process are filtered by the filter networks 28 and 29 which are of the same design and are adapted to eliminate the auxiliary frequency current components and other stray frequencies from their outputs. a

With similar characteristics for the tubes, resistors, and filters, the final output across 3031 is equal to, but in phase quadrature with that existing across 3032 for each frequency present in the original audio source impressed across the terminals 1 and 2. Further, because of the resistance type couplings of the modulators and demodulators, and because the auxiliary frequency of source 3 is not much higher than the highest audio frequency, so that efficient filtering may' be used, the intensity of each output frequency constituent is substantially proportional to the intensity of each input frequency constituent.

Referring now particularly to Fig. 2, this figure shows a circuit for producing a single side band transmission, or transmission of a carrier frequency current and a single side band. The terminals 3032 are used for energizing the grid circuits of the tubes 33 and 34 which comprise a push-pull modulator arrangement, and the terminals 3031 are used to supply the grids of tubes 35 and 36 which comprise a second pushpull modulator circuit.

The details of the two modulator circuits are similar to those of Fig. 1 and willnotbe again described here. The plate circuits of the two theihigh rirequencwcarrier supplied: by@..-the zhigh 33911286. quadrature by means. ofzi tingrcircuit; as; shown.

.The; inductance resistancea of dahe two branches ofithe phase splitter are not the; phase l' split- 1-: E qualibutzare. so: chosen as to :make: the: current through; one :branchizlead the impressed voltage placement'of;

- ers.

1E ".TEL'I'IIEQOU'C of an upper :and .lowerrsid'e band, tbut no carrier ponents upon rthe' gr'id' of amplifier 389: Similarly,

:1:- sithe output of the; modulators iand 36 is im-' r.-.-ap'ressed upon the grid circuit of the amplifier del-vice'39.

Common'to theoutputsrof the amplifiers 38'and -3'9 is thez'resistor it-which servesjt'o combine the 25 outputsofathese amplifiers andimpress the combined voltages upon the grid of the amplifier device-44.

'Voltage of, duced at 37,. is supplied to a" combining tube. 1Q

34736 whichlalso hasacommonplate circuitflwith'the. devices 38- and 39.1 Each of these lastmentioned devices is biased for-amplifi mum distortion.

Ashort circuit switch 53435 ofithe device 40 and serves to control the supply oil-carrier current. to' this. device, and thus,::the radiation of the carrier frequency current. "-''-'.-'I'he "current flowing through resistordl serves into *supply the grid of amplifier 'withgthe. current 40 components to be amplified; :The current thus 2 amplifiedby 44' is emitted fromrth'e antenna. 45

: in'the usual-manner.

A- reversing switch 42 is included in (the: input of the push-pull modulatorv 3536 for therpurpose cation :with a l mini- 45="of reversing,- the direction of flow ofscurrentupon "1 zthe'g'rids ofl35xandi36. a

' "*2? Mathematical proof of the Zeisas follows: I Represent-mat 5,: band" as produ'cedrr by t 'p 'foperation ofz'Fig'. 1

.izrRepresent the'two phase auxiliary current supplied to :the: gridstof 2.4-:and 25 as iz=cos rt and vis sin rt theresult of: thee-combining of these currents after rfiltering out the ,highfrequency will'be;

' seems" (pt-l e) I cuit of Fig. 2 is as'followsg V*1;Bepresentmathematically the two phase speech-,i

band existing across the terminals 30, 31, 32,by two equal amplitude bands as above:

and represent a two phase carrier as supplied by frequency zsource 37;r-the-;ioutput. of? which is so. arranged: asto: deliver the: high frequency: cur parent td 'trieutwo. push-'-pu'll-.:modulator; circuits atndcapacity values.v

byi"45-,tand the other lag by 45 giving disprimary currents ofithe transform put of -the tubes 33 and 34gcons'isting 5 frequency current, isimpressedthroughthe-filter fQMWhiChESGI'VBS tolremove-anyaudiofrequency comthe highcarrier frequency, as pro- 43 isincludedin the'input hematicallycthe-singleupper side pull: modulator aiwhichi lwillbevequal currents but at phase quadraon ofthe-cirlspectively I as By"push-pull-modulation combine the currents a :2 i4- and z'e' likewise' is and 1:7 giv" 'g for a'modulated output the'following:

then by addition and subtractionafterv filtering out the high frequencies produced-by the-com- Fbining: 1 15- Whichrepresent the equations of singlesidegbang 20 transmission bands. 7

With switch 43 ofcircuit in'Fig. 2 closed, and the same constants used for: the tWQ' modulator andamplifier circuits, a sidefrequency existing in resistor 41 as a result of tube 38 will neutralize; 25

the'plate of a like side frequency-'existinginth" output 39; while the'other side frequencies are additive in-effect. Thus there willexist across the resistor l current representative of asingle depends upon the direction of the currentsup plied to the inputs of-the modulator system, and the reversing switch42 may be utilizedtovary the nput or the modulator 355-36 to cause-the current produced, to be representative-of -,eithe,r,-: 35 the'upper or lower side band. 7 N 7 If the carrier is.also tobe .transmitted the switch43 is open sothat current representative of the carrier frequency will then existacross the resistor141. V v Having thus described my invention, attention 7 v is invitedto the .fact that Iv am nottobe limited 7 to the specific form shown and ,describ edfor. the purpose of illustrationpnly, butby the scope of my inventionas set forth in'the appended claims. 5 Iclaimfi I r p M 1. Meansfortheproduction-of twophaseaudio frequency which comprises a source of intermediate frequency energy, a source ofaudiofrequency energy, means formodulating said intermediate energy bysaid audio frequencyenergy, {means for eliminating from-the result of saidm0dulation all except one of the side bands thus produced, and means for independently detecting the audio frequency existing in said side band-by each of twc'phases of the intermediatefrequency-energy, I each of said independent means including two thermionic devices both biased to exhibit plate rectification, an-impedance common to theplate circuit ofboth of said-devices,,and means for impressing the voltage of one of the frequencies involved upon the control-element of one tube and voltage of the other frequency upon the grid of the other tube, 2 .I'he method of producing singlesideband transmission which comprises producing anaudio frequency current representative of therintelligence' to'be transmitted-generating an intermediate frequency current, push-pull modulating the said intermediate frequency current by said 70 audio frequency current, selecting one of the side bands produced by said modulation, splitting the phase of said intermediate frequency current, demodulating the selected side band by combining with each of the phases of said intermediate fre- 7 side band only. Each of the side bands as utilized 30 1 quency current as thus produced, thus producing a two phase audio frequency current, generating a carrier frequency, phase splitting said carrier frequency, modulating one of the phases of said carrier frequency by one of the audio frequency phases thus produced, and eliminating from the result of said modulation the carrier frequency involved, modulating the other phase of said carrier frequency by the other of the audio frequencyphases, eliminating from the result of said modulation the carrier frequency, and combining the product of these two modulations, thereby eliminating one of the side bands produced by each of said modulations.

3. The method of producing single side band transmission which comprises producing an audio frequency current representative of the intelligence to be transmitted, generating an intermediate frequency current, push-pull modulating the said intermediate frequency current by said audio frequency current, selecting one of the side bands produced by said modulation, splitting the phase of said intermediate frequency current, demodulating the selected side band by combining with each of the phases of said intermediate frequency current as thus produced, thus producing a two phase audio frequency current, generating a carrier frequency, phase splitting said carrier frequency, modulating one of the phases of said carrier frequency by one of the audio frequency phases thus produced, eliminating from the result of said modulation a carrier frequency involved, modulating the other phase of said carrier frequency by the other of the audio frequency phases, eliminating from the result of said modulation the carrier frequency, combining the product of these two modulations thereby eliminating one of the side bands produced by each of said modulations, combining the carrier frequency energy with the remaining of the side bands, and transmitting the high frequency components thus produced.

4. The method of producing single side band transmission by the synthetic method which comprises producing a two phase intermediate frequency current, modulating one of the phases of said intermediate frequency current by the audio frequency to be transmitted, selecting one of the side bands thus produced, detecting said side band by each of the phases of said intermediate frequency current thus producing a two phase audio frequency current, generating a carrier frequency, phase splitting said carrier frequency, modulating one of the phases of said carrier frequency by one of the audio frequency phases thus produced, and eliminating from the result of said modulation a carrier frequency involved, modulating the other phase of said carrier frequency by the other of the audio frequency phases, eliminating from the result of said modulation the carrier frequency, and combining the product of these two modulations thereby eliminating one of the side bands produced by each of said modulations.

5. The method of producing single side band transmission by the synthetic method which comprises producing a two phase intermediate frequency current, modulating one of the phases of said intermediate frequency current by the audio frequency tobe transmitted, selecting one of the side bands thus produced, detecting said side band by each of the phases of said intermediate frequency current thus producing two phase audio frequency current, generating a carrier frequency, phase splitting said carrier frequency, modulating one of the phases of said carrier frequency by one of the audio frequency phases thus produced, and eliminating from the result of said modulation a carrier frequency involved, modulating the other phase of said carrier frequency by the other of the audio frequency phases, eliminating from the result of said modulation the carrier frequency, combining the product of these two modulations thereby eliminating one of the side bands produced by each of said modulation, and combining the carrier frequency energy with the remaining of the side bands, and transmitting the high frequency components thus produced.

6. An audio frequency phase splitter which; comprises means for producing an intermediate frequency current, push-pull means for modulating said intermediate frequency current by the audio frequency current which is tobe split, a

filter for eliminatingone of the side bands pro 3 duced by said modulation, means for splitting the phase of said intermediate frequency current, push-pull means for demodulating the side band passed through said filter, each side of said means comprising two thermionic devices having a common output impedance, one of which is adapted to be supplied by one phase of the intermediate frequency current and the other of which is adapted to be supplied by the side band current, and means in connection with said impedance device for producing two phase audio'frequency energy.

7. An audio frequency phase splitter which comprises means for producing an intermediate frequency current, push-pull means for modulating said intermediate frequency current by the audio frequency current which is to be split, a filter for eliminating one of the side bands produced by said modulation, means for splitting the phase of said intermediate frequency current, and push-pull means for demodulating the side band passed through said filter, each side of said means comprising two thermionic devices having a common output impedance, one of which devices is adapted to be supplied by one phase of the intermediate frequency current and the other of which is adapted to be supplied by the side band current.

'8. The method of producing single side band transmission which comprises producing a two phase current of the high frequency, producing a two phase source of the modulating frequency representative of the energy to be transmitted, modulating one phase of said high frequency current by one phase of the low frequency current, modulating the other phase of the high frequency current by the other phase of the low frequency current, combining the result of said modulation so that the phase of one side band will be subtractive while that of the other will be additive,

and also combining the energy of the high fre- 

